What reasons does he give to Sergai for his telling lies?

Sargai had met the beggar earlier and he had introduced himself as a student. When Sargai met the man again, he introduced himself as a teacher. This made Sargai suspect him. After a while of confrontation, the man accepted that he was lying. He told Sargai about getting thrown out of the Russian choir due to his drunkenness. He further told him that he had no other option but to lie and beg because nobody gave him any work. He lied because he knew that telling the truth to people would not get him any help.